There has hitherto been a technique of changing a compression ratio for compressing image data in accordance with a noise amount or a complexity degree of the image data. However, since processing contents become complicated and image processing on the whole of an inputted image is required, load becomes high and hence the foregoing technique has been difficult to install.
Further, in the case of not installing the foregoing technique, fluctuations in code amount due to brightness of a subject's environment are marked. Therefore, even though a frame rate is held when the subject's environment is dark, the frame rate decreases markedly when the subject's environment is bright.
An output signal is feeble when the subject's environment is dark in a typical imaging element. Therefore such control as to set long signal input time in accordance with brightness of an input signal (shutter speed control) is generally performed.
It is to be noted that in the case of a camera sensor (imaging section) having a small area as installed in a communication terminal device such as a cell phone, an original signal is feeble, so that the signal input time widely fluctuates even in a normal photographic environment (e.g. outside, inside, etc.).
Further, in the communication terminal device, with its configuration highly susceptible to hand movement, the influence of the hand movement upon an image increases as the signal input time becomes longer. The hand movement is a contributing factor to image blurring, and consequently, when the subject's environment becomes dark, the degree of image blurring becomes significant.
As opposed to this, a video encoder for use in moving picture photographing, such as MPEG-4, H.263, is capable of transforming an image into a frequency region by DCT (discrete cosine transform) and then intentionally cutting off high frequency components that are contained in small amounts in a natural image, so as to realize a high compression ratio. Therefore, when an image is to be encoded while image quality is held at a given degree, in the case of an image having a large amount of high frequency components, it cannot be efficiently compressed and a code amount per frame becomes large, and on the contrary, in the case of an image having a small amount of high frequency components, only a small code amount per frame tends to be needed.
It is revealed from the above that in photographing a moving picture with a communication terminal device, there is the correlation between the brightness of the subject's environment and the code amount per frame in the video encoder as shown in FIG. 1.
It is therefore desired to apply the correlation between the brightness of the subject's environment and the code amount per frame in the video encoder as shown in FIG. 1, and perform control so as to suppress fluctuations in frame rate due to the brightness of the subject's environment.
It is to be noted that there is a document as a patent document filed prior to the present invention, which discloses a still camera comprising: an imaging means for taking an image of a subject to output digital image data; an aperture means for controlling an amount of light incident to the imaging means; a data compression means for compressing the image data from the imaging means; and a recording medium for storing the image data compressed by the compression means, wherein a compression ratio control means is provided for controlling a compression ratio of the data compression means based on an aperture value of the aperture means, and in the case of automatically selecting a compression coefficient for data compression, the compression ratio can be selected with low power consumption at high speed (cf. e.g. Patent Document 1).    Patent Document 1: Japanese Patent Laid Open Publication No. H7-177463